1. Field of the Invention
Polyphenylene oxides ("PPOs") and processes for their production are known. They are described, e.g., in Buehler, 1978, "Special Plastics", pub. Akademieverlag, Berlin (in German), and in U.S. Pat. Nos. 3,306,874 and 3,306,875.
2. Description of the Prior Art
Processes have been employed industrially (and have proven economically feasible) in which ortho-disubstituted phenols are oxidatively linked with the aid of copper-amine complexes, in the presence of oxygen or oxygen-containing gas mixtures. In these known processes, three process steps are necessary from the end of the reaction to the production of the end product in powder form. These are namely; the interruption of the polycondensation, the purification of the polymer, and the isolation of the PPO.
The polycondensation may be advantageously interrupted (depending on the reaction conditions) by either inactivating the catalyst or by separating out the catalyst or the polymer. However, in particular cases these measures, while they indeed interrupt the polycondensation, also result in incomplete purification of the polymer.
The most important interruption methods are the following: 1. Addition of aqueous acids; 2. Addition of alkalis; 3. Addition of complex-formers for copper ions; and 4. Addition of polar solvents for separating the polymer.
Probably the most frequently employed method of interrupting the polycondensation is the addition of aqueous acid solutions. However, the consumption of acid is substantial, and the degree of removal of the amine is unsatisfactory (see Ger. OS No. 22 46 552, p. 4, lines 1 to 8). Carrying out the operation in an extraction apparatus, e.g. countercurrent to a stream of aqueous acid, and possibly at elevated temperature (Ger. OS No. 21 05 372), represents a process advance, but also necessitates the use of large amounts of alkali metal hydroxides to recover the amines.
According to the process of the above-cited Ger. OS No. 22 46 522, the PPO reaction mixture containing the copper-amine catalyst and the polymer in an aromatic solvent is subjected to interruption by treatment with CO.sub.2 in water, and then undergoes further processing. The amines are not separated from the polymer to a satisfactory degree, as seen in Example A, infra.
In Ger. Pat. No. 15 70 683, a process is described in which a 50% solution of sodium hydroxide is added to the PPO reaction mixture. It has not been possible to make this process commercially successful because the polymerization is not reliably interrupted (see Ger. Pat. No. 24 30 130).
With the aid of chelating agents, the catalyst is inactivated and the copper separated out (see Ger. Pat. No. 15 70 683, col. 15, lines 3 to 5; and Ger. OS No. 26 40 147). Additional experimental details may be obtained from Ger. OS No. 23 64 319. The critical disadvantage of all processes employing chelating agents is the reported decomposition of the polymer. It has been found (see Ger. OS No. 27 54 887) that the higher the temperature the more rapidly the polyphenylene oxide decompose; and the longer the interval between the end of the reaction and the isolation of the polymer the greater the degree of decomposition of the polyphenylene oxides. For example, the viscosity index (which is also an index of the average molecular weight of the polymer units) commonly decreases by more than 0.1 dl/g/hr at 50.degree. C., and fairly often by 0.2 dl/g/hr or more (Ger. OS No. 27 54 887).
When special chelating agents are used, e.g. the polyamines of the process disclosed in Ger. OS No. 24 60 323, this decomposition remains unavoidable.
It has been proposed to add additional stabilizing agents such as bifunctional phenols (e.g. dihydroxybenzenes) and reducing agents (see Ger. OS No. 27 54 887) or aromatic amines (see Ger. OS No. 27 55 937) to the polymer solution in addition to the chelating agent. These solutions with the additional additives still cannot yield a satisfactory result, because fairly high concentrations of the additives are required, which in itself is a drawback and which further leads to additional difficulties in isolating the polyphenylene oxides.
So-called "antisolvents"--alcohols, ketones, or aqueous solutions of these--may be availed of, whereby the PPO polycondensation can be interrupted and the polymer separated out, while catalyst residues and diphenoquinone remain in the solvent. Nonetheless, small amounts of copper may remain in the polymer, which detrimetally effects the color of the polymer and catalyzes further decomposition (see Ger. OS No. 26 16 746).
A drawback common to all of the precipitation methods (e.g., Ger. Pat. Nos. 12 65 410 and 15 70 683, and Ger. OS Nos. 25 32 477, 25 32 478, 26 55 161, and 27 52 867) is the need to employ, process, and redistill large amounts of solvent (with accompanying losses). Therefore it is preferable in practice to recover the polymers by means of a so-called "direct isolation" process, e.g. steam stripping, spray drying, or hot water comminution. However, a precondition for the use of such processes is the prior removal of the residues of copper-amine catalyst and other impurities, to the extend quantitatively possible (see Ger. OS No. 24 60 323).
Finally, there are a number of known processes for improving the color characteristics of polyphenylene oxides, wherein reducing agents are employed. Thus, for example, Ger. Pat. No. 24 30 130 teaches the application of a dihydroxybenzene or a benzoquinone and a mild reducing agent. A drawback is the requirement for large amounts of dihydroxybenzenes, which are relatively un-biodegradable; and also the large amount of time required for separating the reaction mixture from the aqueous solution employed. Accordingly, thus far it has not been possible to develop these processes in economically feasible form.